Natural gas accounts for approximately 20% of the world energy consumption and is the third largest used fuel after oil and coal [1]. The United States is the second largest producer of natural gas in the world, with an annual production of 546 billion cubic meters (bcm). In addition to the domestic production, the U.S. is also the largest importer of natural gas, at 130 bcm per year [1]. Natural gas prices have steadily increased over the past decade. Synthetic natural gas produced from coal at competitive prices will be an attractive option since this can be accomplished using domestic feedstocks. The production of methane from coal for use as synthetic natural gas (SNG) has been studied for many years and the interest was especially high during the 1970s and 80s.
Limitations of the Conventional Processes for SNG Production
Hydrogasification: The primary thermo-chemical process used for the production of synthetic natural gas from coal was hydrogasification. Hydrogasification was originally developed in the early 1900s and there was a revived interest in the process during the 1970s and 80s as a result of increasing natural gas prices. The basic reaction is the direct methanation of carbon, as shown below [2].C+2H2CH4 ΔH1000 K=−89.9 kJ/mol  (1)
Although this reaction is mildly exothermic, significant amount of energy must be spent in bringing the reactants up to temperature and also to sustain the process. Methane production is favored at high pressures and the process is generally operated at temperatures ranging from 750° C. to 1000° C. [3]. A number of processes were developed and a few of these were operated satisfactorily in pilot plant scales. A major issue with hydrogasification processes was the source of hydrogen supply since hydrogen production can be expensive. Natural gas prices also dropped during this period. In addition to the hydrogen supply issues and cheap natural gas, hydrogasification was not very attractive due to the much slower reactivity of carbon with hydrogen compared to other gasifying agents. The reactivity of carbon with different species at 1073 K and 0.1 atmospheres are shown below [4].
            r              O        2                    10      5        >>                    r                              H            2                    ⁢          O                    3        >                  r                  CO          2                    1        >                  r                  H          2                            3.1                  -          3                    
Methanation of Partial Oxidation Product gas: Oxygen or air blown processes are the primary focus of current gasification development, especially in commercial and large scale demonstration projects. The oxygen blown processes are commonly known as partial oxidation (POX) technologies and offer high carbon conversions under low residence times in the reactor. However, these processes generate a synthesis gas (syngas) with very low methane content. Hence, these processes can be used for methane production only by means of downstream methanation. Methanation processes are not considered to be an effective means of synthetic natural gas production due to different reasons including relatively poor efficiency under desired process conditions [5].
Based on the above discussions, it is evident that for SNG production to be commercially viable, the gasification process must solve the two major technical problems faced by conventional hydrogasification processes and methanation process. These problems are the difficulties in the supply of hydrogen in an inexpensive and simple manner and also the low carbon conversions observed during conventional hydrogasification based processes.